In recent years, higher integration and higher density of electronic parts have elevated electric power consumption per chip. Thus, effective removal of the generated heat in order to suppress temperature elevation of electronic elements is a critical issue in the development of electronic elements. In view of the foregoing, alumina, particularly corundum (α-alumina), exhibiting excellent thermal conductivity, has become a candidate filler for a heat-dissipation spacer; a substrate material on which insulating sealing materials for semiconductors and parts of semiconductor devices are mounted; etc., and has been used in a variety of fields.
Among such corundum particles, JP-A HEI 5-294613 discloses spherical corundum particles having no fractures, the particles being produced by adding aluminum hydroxide and optional, known agents serving as crystallization promoters in combination to a pulverized product of alumina, such as electrofused alumina or sintered alumina, and firing the mixture.
There has been also known a thermal spraying method in which alumina produced through the Bayer method is atomized into high-temperature plasma or oxygen-hydrogen flame so as to melt and quench, to thereby produce roundish particles. The unit heat energy requirement of the thermal spraying method is large, thus not economical. In addition, the thus-produced alumina, though predominantly containing α-alumina, includes by-products such as δ-alumina and has low thermal conductivity. Therefore, such an alumina product is not preferred.
Pulverized products of electrofused alumina or sintered alumina have been known as corundum particles. However, these corundum particles are of indefinite shape having sharp fractures and produce significant wear in a kneader, a mold, etc. during incorporation thereof into rubber/plastic. Thus, these corundum particles are not preferred.
According to the method disclosed in the aforementioned publication, round-shaped corundum particles having no fractures and a mean particle size of 5 to 35 μm can be produced. However, the method poses some problems in relation to production of large amounts of such corundum particles at low costs on an industrial scale.
The method disclosed in the above publication includes adding one or more species selected from among a halogen compound, a boron compound and an alumina hydrate to a pulverized product of electrofused alumina and/or sintered alumina having a predetermined particle size and heating the resultant mixture at 1,000 to 1,550° C. When this method is employed, the fired product is strongly solidified to form aggregates in a container for firing.
The above publication also discloses further addition of an alumina hydrate to reduce the hardness of the aggregates. However, the effect is unsatisfactory. Particularly when a large container for firing is employed in order to produce large amounts of corundum particles on an industrial scale, the resultant fired aggregates become commensurate to the size of the container for firing. When the aggregates are crushed and pulverized, multi-step crushing must be carried out, resulting in considerably high costs.
Moreover, since the fired product is strongly adhered to the inner surface of the container for firing, removal of the fired product is difficult, requiring additional treatment such as application of mechanical stress. In this case, considerably large stress is applied to the container for firing, which may cause breakage of the container itself. Thus, this method is not satisfactory in terms of economy.
The present inventors have carried out extensive studies in order to solve the problems in relation to the aforementioned conventional techniques, and have found that, in the production process for spherical corundum particles, a mixture composition is granulated prior to heat treatment and the resultant granules are fired to thereby solve the problems. The present invention has been accomplished on the basis of this finding.